In recent years, a shift-by-wire system is generally used in a vehicle. For example, as disclosed in U.S. Pat. No. 6,857,981 (JP-A-2004-52928), a shift-by-wire system includes an electric motor for actuating a range switching unit to switch a shift range of an automatic transmission device, in accordance with an instructed shift range selected by a vehicular occupant.
In switching of the shift range of the automatic transmission device, a torque required for actuating the range switching unit differs depending upon the shift range to be switched. Specifically, the required torque becomes the maximum when the shift range is switched from a P range to another range, such as an R range. In particular, when the shift range is switched from the P range to the other range in a condition where the vehicle is parked on a sloping road, a torque required for bringing a parking lock mechanism into the unlocked state thereof becomes large. In this state, the required torque becomes still further greater.
Therefore, the shift range switching device disclosed in U.S. Pat. No. 6,857,981 includes a reduction device for transmitting the power of the electric motor to the range switching unit. The reduction device is selected to be in one gear ratio for producing the torque required in the switching from the P range to the other range.
The parking lock mechanism is provided for bringing the output shaft of the automatic transmission device into the locked state thereof in conjunction with the switching actuation of the range switching unit to the P range. Alternatively, parking lock mechanism brings the output shaft of the automatic transmission device into the unlocked state thereof in conjunction with the switching actuation to the other range.
In U.S. Pat. No. 6,857,981, the P range can be reliably switched to the other range when the gear ratio of the reduction device is selected to produce the torque required in the switching from the P range to the other range. However, the range switching unit is actuated at a large gear ratio, even in any other switchings when such a large torque is not necessary. Consequently, actuations of the range switching unit in the other switchings becomes slow, and response of the range switching unit is impaired.